| Aqueous lubrication due to its advantages,such as no pollution to environment,accessibility,energy savingand natural coolant,has become one of the most promising lubricant medium,there related research used in the field of tribology has aroused widespread concern.Actually,it is impossible to form an effective lubricating film,mainly because pure water has very low viscosity and pressure-viscosity coefficient,which makes aqueous lubrication not applied widely especially in industry.Interestingly,water is a good lubricant in living organisms in nature.For example,the synovial joint has a friction coefficient of less than 0.003 and a service life of several decades years,mainly due to the "bottle-like" polymer brush,which acts as a good load bearing and enhances the lubrication properties that cannot be achieved due to the low viscosity of water.It was found that these polymer brushers grafted to the rubbing surface can reduce effectively the friction and wear,and there is a series of very valuable work conducted on micro-scale.In recent years,researchers have started to pay attention to tribological performance research on a macro scale.Based on this,this work was carried out as the following:Firstly,polymer brushes,named as PLL-g-PEG and PSPMA respectively,were grafted to the rubbing surfaces made of steel and glass by 2 preparation methods of"grafted to" and "grafted from";Secondly,the effect of polymer brushes on surface wetting properties were characterized by means of contact angle meter;Thirdly,friction was measured on ball-3 plate friction testing module on rheometer(Anton Paar Physica MCR 302)and ball-disc friction tester(UMT-3)to evaluate the effects of polymer brushes on friction under aqueous lubrication enviornment;after the friction testing,scanning electron microscopy(SEM)and contact surface profilometer PGI-800 were used to obtain the surface topography information of the worn scars;aqueous lubrication behavior caused by polymer brush was in situ observed between a steel ball and glass disc grafted with polymer brush using dichromatic interference intensity modulation approach,and then the effect of polymer brushes on film formation properties was investigated,the relevant measurement results show as the following:(a)It was found that the addition of PLL-g-PEG to an aqueous solution of N-(2-hydroxyethyl)piperazine-N’-2-ethanesulfonic acid(HEPES)(0.25 mg/mL)can lead to reduce effectively the friction and wear,and this improvement was mainly affected by contact stress,entrainment speed and substrate material;ultra-low friction(coefficient of friction is about 0.003)can be obtained at lower contact stress,higher entrainment speed and hydroxyl-rich substrate surface,hopefully which may guide to designing surface materials for low-friction;(b)It was confirmed the "self-healing" behavior of polymer brush PLL-g-PEG under aqueous lubrication environment by our experiment,i.e.polymer brush PLL-g-PEG was continuously destroyed and peeled off the surface by shear stress during the rubbing time,simultaneously it can rapidly graft to the substrate surface under the action of static electricity,and then a dynamic balance was achieved after a relatively short period of time,It was the "self-healing" behavior that lead to the friction effectively decreased,here it was revealed how the polymer brush PLL-g-PEG reduce friction and wear;(c)The results of friction and wear measurement under water lubrication environment showed that under milder conditions,such as at the load of 0.5 N and entrainment speed 10 mm/s,polymer brusher PSPMA reduced friction and wear to a certain extent,while it had little effect on reducing the friction and wear.Under harsh conditions,such as load 3.5 N and entrainment speed 300 mm/s,it indicated that PSPMA exhibited poor tribological properties compared with PLL-g-PEG,which may ascribed to that PSPMA cannot re-grafted to the rubbing surface and then to fail after it was destroyed;(d)When polymer PLL-g-PEG was added to HEPES-glycerol aqueous solutions with different concentrations of glycerol(10%V/V,25%V/V and 50%V/V,respectively),results showed that higher concentration of glycerol concentration,the worse tribological performance of polymer brushes.This is because glycerol was bad solvent for polymer brushes,and polymer brusher can exhibit different conformations in different solvents,such as,polymer brush had stretching brush-like structure due to hydration and then effectively reduced friction in HEPES with 10%V/V glycerol concentration,while this structure collapsed in the 50%V/V concentration of HEPES-glycerol aqueous solution,resulting in weak impact on friction,which suggests that macro-friction properties may be achieved by adjusting the conformational changes of polymer brushes;(e)Optical interference results showed that a very thin hydration layer could be formed between the steel ball and glass disc grafted polymer brush PLL-g-PEG,which improved the lubrication to a certain extent,but the effect is not ideal due to its limited carrying capacity.Under the same experimental conditions,polymer brush PSPMA obtained much higher film thickness compared with the polymer brush PLL-g-PEG,which means that polymer brush PSPMA has exhibited excellent film-forming performance,it may be due to its higher bearing capacity and then effectively separated from the two contact surfacesThis work comprehensively studied the effects of polymer brushes(PLL-g-PEG and PSPMA)grafted by different preparing methods on the macro-tribological properties under aqueous lubrication environment and then investigated their lubrication behavior based on interference technology.The film-forming mechanism of aqueous lubrication was further revealed,which provided necessary technical support for guiding to design the polymer lubricating materials.It is important to expand the application of aqueous Lubrication lubricants and to meet the acquirement to develop green lubrication. |
PDF Full Text Request